The present invention generally relates to durable thermal barrier coatings and methods for making the same. More particularly, the present invention relates to a durable thermal barrier coating having improved bonding to a bond coating on a substrate.
Ceramic thermal barrier coatings (TBCs) have received increased attention for advanced gas turbine engine applications. The advantages of using TBCs include increased engine efficiency by allowing higher gas temperatures and improved reliability from lower component temperatures. TBC systems have been more aggressively designed for the thermal protection of engine hot section components, thus allowing significant increases in engine operating temperatures, fuel efficiency and reliability. However, the increases in engine temperature can raise considerable coating durability issues. The development of next generation lower thermal conductivity and improved thermal stability TBCs thus becomes a necessity for advancing the ultra-efficient and low emission gas turbine engine technology.
Partial loss of cohesion between an yttria stabilized zirconia (YSZ) TBC deposited by, for example, electron beam physical vapor deposition (EB-PVD), and an underlying bond coating may contribute to TBC spalling. When this partial loss of cohesion occurs, alumina growth stresses and alumina-superalloy thermal expansion mismatch stresses within the thermally grown oxide, which occur during thermal transients, may form microbuckles in the thermally grown oxide at the TBC-bond coating interface. Once initiated, interfacial microbuckles continue to grow at operational temperatures in the range of 900 to 1150° C. because bond coatings, such as Pt-aluminide or NiCoCrAlY, have insufficient creep-strength to constrain the area-growth of the thermally grown oxide scale.
U.S. Patent Publication Number 2003/0203221, by Spitsberg, discloses a thermal barrier coating that may be improved by thermally preoxidizing the grit blasted single-phase Pt-aluminide bond coating in a low pressure vacuum prior to deposition of the thermal barrier coating. Preoxidation in vacuum forms a thin layer of pure alumina on the Pt-aluminide bond coating. Spitsberg has the disadvantage that the preoxidation parameters are specific to a particular substrate and bond coating. Furthermore, Spitsberg states (at paragraph [0025]) that the preferred heat treatments produce a crystallographically stable alpha-alumina, which maximized the TBC life in furnace cycle tests.
U.S. Pat. No. 6,482,537, to Strangman et al., discloses the reduction of the thermal conductivity of a TBC by decorating deposition interfaces within the columnar grains of an EB-PVD thermal barrier coating with nano-scale particles of alumina or tantala. The Strangman patent discloses that the bond coating may have a thermally grown alumina scale and that thermal growth of the alumina scale may be performed prior to or during EB-PVD deposition of the TBC.
U.S. Pat. No. 6,485,844, to Strangrnan et al., discloses a fully metastable high purity alumina scale that may be thermally grown at low temperatures on an undiffused bond coating comprising a high purity layer of platinum and a CVD deposited layer of high purity alumina. The Strangman patent discloses that, under controlled conditions, a metastable alumina scale is grown at low temperature and may be subsequently converted to the stable alpha phase during a high-temperature heat treatment prior to the deposition of the TBC.
As can be seen, there is a need for improved thermal barrier coatings having long life and high reliability, whereby TBC spalling is minimized or eliminated. There is also a need for an improved method for making such a thermal barrier coating that would resist spalling over repeated temperature cycles.